Avoid calling slower Node::firstChild() / Node::lastChild() when possible

Source/core/dom/Node.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2008 Nokia Corporation and/or its subsidiary(-ies)
  * Copyright (C) 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
@@ skipping 1467 matching lines @@
     case Node::ELEMENT_NODE:
         if (isHTMLBRElement(*node) && convertBRsToNewlines) {
             isNullString = false;
             content.append('\n');
             break;
         }
     // Fall through.
     case Node::ATTRIBUTE_NODE:
     case Node::DOCUMENT_FRAGMENT_NODE:
         isNullString = false;
-        for (Node* child = node->firstChild(); child; child = child->nextSibling()) {
+        for (Node* child = toContainerNode(node)->firstChild(); child; child = child->nextSibling()) {
             if (child->nodeType() == Node::COMMENT_NODE || child->nodeType() == Node::PROCESSING_INSTRUCTION_NODE)
                 continue;
             appendTextContent(child, convertBRsToNewlines, isNullString, content);
         }
         break;
 
     case Node::DOCUMENT_NODE:
     case Node::DOCUMENT_TYPE_NODE:
         break;
     }
@@ skipping 1097 matching lines @@
         node->showTreeForThis();
 }
 
 void showNodePath(const WebCore::Node* node)
 {
     if (node)
         node->showNodePathForThis();
 }
 
 #endif